The present inventor developed a low temperature type system of neuro-magnetic field sensors in which a SQUID can be used while being immersed in a liquid helium bath. The instrument has been practically used.
Referring to FIG. 5, a conventional SQUID (MagnetoEncephaloGraphy or MEG) apparatus 1 comprises a vacuum structure 11 of hollow cylinder for thermal insulation, a closed-cycle Helium refrigerator 12, a liquid-Helium dewar 13, and a top cover 14. The vacuum structure 11 for thermal insulation of a hollow cylinder contains two cylinders—a first, inner cylinder 111 of high critical temperature superconductor material and a second, outer cylinder 112 of high-permeability magnetic material both arranged coaxially in its annular space. The closed-cycle Helium refrigerator 12 circulates a cooled Helium gas to cool the inner cylinder of high critical temperature superconductor material in the vacuum structure 11 for thermal insulation. The liquid-Helium dewar 13 is arranged coaxial to the vacuum structure 11 for thermal insulation. The top cover 14 is of double structure of a metal of electrically conductive material (taking part of shielding electromagnetic wave) and a magnetic material (taking part of shielding magnetic field), and is adapted to fit the top of the vacuum structure 11 for thermal insulation.
The lower part of the liquid-Helium dewar 13 defines a head accommodating area 131 to accommodate the head of a patient under inspection. The liquid-Helium dewar 13 has a plurality of SQUID magnetic sensors 15 therein. The SQUID magnetic sensors 15 are fixedly arranged on a support block 20 around the head accommodating area 131. The liquid-Helium dewar 13 is filled with liquid Helium of cryogenic temperature.
The vacuum structure 11 for thermal insulation is supported by the horizontal shafts on the four legs. It has a non-magnetic chair 17 placed in its lower opening. The top cover 14 of magnetic material is put on the top of the vacuum structure 11 for thermal insulation, effectively preventing invasion both of the geomagnetism and the electromagnetic wave from the top.
The conventional MEG apparatus is described in the following documents: (Patent Document)
Patent Application Public Disclosure No. 10-313135; and (Non-Patent Document)
“Whole-Head-Type SQUID System in a Superconducting Magnetic Shield of High Critical-Temperature Superconductor”, by Hiroshi Ohta, “Ceramics 35” (2000), No. 2. Extra Edition. Titled “Brain and Ceramics; Ceramics Useful in Illustrating the Functions of the Brain, Making the Diagnosis of the Brain Disorders and Carrying out Required Treatments”, and
“Nanometer SNS Junctions and Their Application to SQUIDs”, by Hiroshi Ohta et al, “PHISICA C” 352 (2001), p.p. 186-190
Conventionally it is used to be necessary that the SQUID (MEG) system be completely isolated from the floor of a building to avoid mechanical vibrations. The complete isolation of the SQUID (MagnetoEncephaloGraphy or MEG) system from any mechanical vibrations requires rigid floors of the building usually. Also, an optimum installation site should be chosen to avoid mechanical vibrations from the surroundings such as traffic of automobiles; if not, the MEG system installed in the existing building could not be of practical use. When a building which a MEG system is to be installed in is constructed, the solid underground base of the building needs to be rigid and strong enough to shut off any mechanical vibrations from the surroundings, and accordingly the required foundation work takes much money to build.
Referring to FIG. 6, a MEG system was installed in an existing building with a rigid foundation, and the signals from the typical fifteen SQUID magnetic sensors 15 among 64 sensors of the system were plotted with time (abscissa). As seen from these records, most of 15 channels have significantly large noise signals while no patient was under inspection. At the outset we were not able to identify sources of such noise signals, and it took some time before we recognized that the source of such significant noise signals is constant, ceaseless vibrations of minimum amplitude from the floor.
FIGS. 7 and 8 show the vacuum structure 11 for thermal insulation and the liquid-Helium dewar 13 at an enlarged scale. Referring to these drawings, assuming that the first cylinder 111 of the high critical temperature superconductor material (bismuth-strontium-calcium-copper-oxides: BSCCO) is cooled down to around the liquid nitrogen temperature (below Tc=103 K), invasion of magnetic flux in the inner space of the vacuum structure 11 for thermal insulation would be supposed to be completely prevented. Before the first cylinder 111 is cooled down, however, the geomagnetic field has already invaded into the inner space of the vacuum structure 11 for thermal insulation, and then, the geomagnetic field is pin-fastened to the first cylinder 111 in the state of being trapped. In this position if the liquid-Helium dewar 13 moves relative to the first cylinder 111 longitudinally or up and down (see FIG. 7) or laterally or from side to side (see FIG. 8), the magnetic component of the trapped static magnetic field across the SQUID sensors 15 will vary (see FIGS. 7 and 8; magnetic fluxes and sensors after displacement being shown by broken lines), thereby causing noise signals to appear.
One object of the present invention is to provide a noise-free MEG apparatus of high-sensitivity. Another object of the present invention is to provide a method of putting such MEG apparatus in operation.